1. Field of the Invention
The present invention relates to a communication apparatus and a coexistence method for enabling coexistence of communication systems. More particularly, the present invention relates to a technique of enabling coexistence of two communication systems which use the same communication medium and have different communication schemes (data communication is performed while maintaining AV-QoS (Audio Video-Quality of Services) required for video transmission and audio communication), a communication apparatus included in each of the communication systems, and a coexistence method which is executed by the communication apparatus.
2. Description of the Background Art
Power line communication technology is a communication means for connection of a Personal Computer (PC) in a home to a network apparatus, such as a broadband router or the like, so as to access from the PC to the Internet. In the power line communication technology, since an existing power line is used as a communication medium, it is not necessary to perform a new wiring work, and high-speed communication can be achieved only by inserting a power supply plug into a power supply outlet available throughout a home. Therefore, research and development, and demonstration experiments of the power line communication technology have been vigorously conducted all over the world, and in Europe and the USA, a number of power line communication projects have already been commercialized.
An example of the power line communication is HomePlug Ver. 1.0 (see Yu-Ju Lin, “A Comparative Performance Study of Wireless and Power Line Networks”, IEEE Communication Magazine, April, 2003, pp. 54-63), which is a specification created by the HomePlug Powerline Alliance (USA). The specification is intended to be used mainly in applications, such as the Internet, mailing, and file transfer which are performed by PCs. HomePlug employs a CSMA/CA technique for a medium access control of which power line communication modem accesses a power line, and provides best-effort communication which does not guarantee a band to be used.
FIG. 18 is a diagram illustrating a configuration of a general communication system when accessing the Internet. In FIG. 18, a PC 2501 is connected via an Ethernet 2511, a broadband router 2502, and an access line 2512 to the Internet 2522. As the access line 2512, ADSL, FTTH, or the like is generally used. Here, when a place where the access line 2512 is withdrawn into a home is different from a room where the PC 2501 is placed, the Ethernet 2511 needs to be extended. Therefore, a power line communication apparatus has been commercialized in the form of a conversion adaptor between power line communication and Ethernet.
FIG. 19 illustrates a configuration of a communication system employing a conversion adaptor. In FIG. 19, two power line communication-Ethernet conversion adaptors 2603 and 2604 are connected to power supply outlets in rooms where a PC 2601 and a broadband router 2602 are installed, respectively, and provide best-effort communication by using power line communication via an in-home power line 2614. Thus, by using power line communication, wiring work is not required, and high-speed communication can be achieved only by inserting a power supply plug into a power supply outlet available throughout a home.
In Europe (Spain, etc.), an access power line communication modem has been used which employs, as an access line to the Internet, a power line for supplying a power to a home. FIG. 20 is a diagram illustrating a situation where the access power line communication modem is used. An access power line communication modem master station 2703 provided at an outdoor transformer, is connected via an intermediate voltage power distribution line 2713 to a broadband line, and communicates with an access power line communication in-home modem 2702 via a low voltage power distribution line 2712, a distribution switchboard 2715, and an in-home power line 2711. Further, by connecting the access power line communication modem 2702 with a PC 2701 via an Ethernet 2704, access to the Internet can be performed from the PC 2701.
Thus, by using the access power line communication modem, access to the Internet can be provided without withdrawing a cable or the like into a home. In addition, since the access power line communication modem 2702 is installed at any arbitrary outlet in a home, the degree of freedom of installing is higher than that of ADSL, FTTH, and the like.
FIG. 21 is a diagram illustrating an internal configuration of a general power line communication modem which is implemented as a bridge to Ethernet. In FIG. 21, the power line communication modem comprises an AFE (Analog Front End) 2801, a digital modulation section 2808, a communication control section 2809, and an Ethernet I/F section 2810. The AFE 2801 includes a BPF (Band-Pass Filter) 2802, an AGC (Automatic Gain Control) 2803, an A/D conversion section 2804, an LPF (Low-Pass Filter) 2805, a PA (Power Amplifier) 2806, and a D/A conversion section 2807. Hereinafter, an operation of the power line communication modem will be described.
Assuming that Ethernet frames are transmitted onto a power line, when an Ethernet frame arrives through an Ethernet 2811, the communication control section 2809 is notified of the arrival via the Ethernet I/F section 2810. The communication control section 2809 determines a state of a communication channel, and outputs frame data to the digital modulation section 2808 with appropriate timing. The digital modulation section 2808 performs error correction addition, encoding, framing, and the like to modulate the frame data into a transmission data sequence. The D/A conversion section 2807 converts the transmission data sequence from a digital signal to an analog signal. The PA 2806 amplifies the analog signal. The LPF 2805 cuts off signals other than communication band components from the amplified analog signal, and inputs only the communication band components onto a power line. Next, in the case of reception from a power line, the BPF 2802 extracts a signal in a communication band. The AGC 2803 amplifies the extracted signal. The A/D conversion section 2804 converts the amplified analog signal into digital data. The digital modulation section 2808 performs frame synchronization detection, equalization, decoding, error correction, and the like with respect to the digital data to demodulate the digital data and notifies the communication control section 2809 of the resultant data as reception data. Thereafter, the reception data is transmitted as an Ethernet frame from the Ethernet I/F section 2810 to the Ethernet 2811.
Although the first-generation technology for high-speed power line communication is intended to be applied to best-effort applications, such as mailing and Web access on the Internet, power line communication for which outlets are provided everywhere in a home (i.e., wiring is not newly required) has potential to allow VoIP and video distribution, which are becoming more digital, to be used everywhere in a home.
However, in the case of VoIP, a sense of discomfort occurs as a delay time increases in voice signals. Therefore, a packet whose transmission delay exceeds a predetermined level is discarded. The packet discarding leads to loss of audio information. As the frequency of the packet discarding increases, discontinuity or noise occurs in audio. On the other hand, in the case of video distribution, a large amount of data needs to be communicated. For example, in the case of high-definition video, the data amount per second is as large as 24 Mbits. Such a large amount of data needs to be transmitted by an apparatus with a delay time which is within a tolerable range. This quality requirement for transmission of AV data is called AV-QoS, which is generally defined by an average transmission rate, a delay time, jitter, or the like.
Conventionally, as a technique of simultaneously achieving both power line communication satisfying AV-QoS and best-effort power line communication, a hybrid medium access control method of TDMA (Time Division Multiple Access) and CSMA (Carrier Sense Multiple Access) has been proposed. FIG. 22 illustrates an example of the conventional hybrid medium access control method.
In FIG. 22, the power line communication system which is designed to satisfy AV-QoS is composed of one terminal having a master function (master terminal) and one or more terminals having a slave function (slave terminals). The master terminal transmits a beacon 1201 at a predetermined time. The predetermined time is referred to as a beacon cycle. As the beacon cycle decreases, a data delay can be suppressed to a smaller level, but a data amount transmitted by one packet decreases, so that the proportion of overhead, such as header information and the like, increases, resulting in a decrease in transmission efficiency. In general, the beacon cycle is set to be about 10 msec to about 100 msec in view of a delay time requirement and transmission efficiency of a transmission signal. Also, in the beacon, time regions in which communication is permitted for the respective terminals are described. In the example of FIG. 22, terminals #1 to #3 are permitted for respective predetermined times. Also, the master terminal allocates a CSMA period, following TDMA in which transmission is controlled every a predetermined time. During the period, a terminal having a transmission signal acquires a transmission right using a predetermined algorithm to transmit data. Therefore, the period is suitable for transmission of conventional Internet data, such as mailing and Web access, in which a predetermined amount of data is not generated at every predetermined time. However, once data transmission is started, data communication is continued in a burst manner. By separating from the TDMA region in this manner, it is possible to avoid local interruption of data transmission in the TDMA region.
Next, a coexistence control for power line communication will be described.
As described above, various forms of power line communication from a home to an access network have been considered, and various types of power line communication techniques have been developed, but there is currently no unified power line communication scheme. However, in-home power lines are all connected together in a distribution switchboard, and are also connected with an outdoor power line. Therefore, if power line communication modems for different schemes are used in the same home and outside the home (close to the home), a communication signal from one modem is likely to reach other modems. A power line communication modem for one scheme cannot demodulate a signal of another scheme which a power line communication modem for the other scheme transmits in a communication channel, i.e., the signal of the other scheme is only noise. Therefore, if two different communication schemes are simultaneously performed, the two schemes interfere with each other, so that neither of the two schemes achieves communication, resulting in a significant decrease in communication speed, or the like.
As a method for avoiding such a problem, it is considered that a unified standard scheme for power line communication is created. However, a huge time and cost are required to create a new standard, so that such a standard will not be obtained in the near future.
To avoid this, for example, Japanese Patent Laid-Open Publication No. 2002-368831 proposes a method for controlling data transmission of each power line modem when a plurality of power line modems having different data communication schemes are present on the same power line. FIG. 23 is a diagram for explaining this conventional technique.
In FIG. 23, for example, it is assumed that a selector 61 provided in a management processor 6 selects power line modems 4a to 4m employing a scheme B as transmission-permitted power line modems. In this case, a message generator 62 generates a transmission-permitting message which indicates permission of transmission to the power line modems 4a to 4m employing the scheme B, and a transmission-forbidding message which indicates forbiddance of transmission to power line modems 3a to 3m employing a scheme A. Thereafter, a power line modem 3n employing the scheme A transmits the transmission-forbidding message to the power line modems 3a to 3m employing the scheme A, and a power line modem 4n employing the scheme B transmits the transmission-permitting message to the power line modems 4a to 4m employing the scheme B.
FIG. 24 illustrates an operation in which two in-home systems coexist on the same power line in a time division manner using the communication system of FIG. 23. In FIG. 24, the management processor 6 outputs a coexistence signal 1401 which permits an in-home communication system 1 to perform transmission, and a coexistence signal 1402 which permits an in-home communication system 2 to perform transmission. By periodically repeating the operation using predetermined TDM, equal time slots are allocated for the in-home communication system 1 and the in-home communication system 2 without collision.
However, in the above-described conventional system, AV-QoS cannot be secured between a plurality of communication systems. Specifically, a communication system which transmits the coexistence signals 1401 and 1402 is different from a communication which tries to secure AV-QoS in a home. When different communication systems transmit a coexistence signal, clocks of the communication systems are not in synchronization with each other, and therefore, the beacon 1201 of the in-home communication system which should secure AV-QoS has a phase that is different from that of the coexistence signal, so that the phase difference increases over time.
Therefore, as illustrated in FIG. 24, even if the beacon 1201 is transmitted in a first slot of the in-home communication system 1 during a coexistence cycle immediately after a given coexistence signal, a beacon is shifted afterward in a slot of the in-home communication system 1 during a coexistence cycle after a lapse of some cycles. Therefore, a transmission time of a terminal in the in-home communication system 1 which starts transmission at a time designated by the beacon 1201 is shifted into a time slot of another coexisting system, so that both systems collide, and therefore, a coexistence relation cannot be maintained. In other words, AV-QoS cannot be maintained.
Note that, since an access communication system provides service to a number of homes, the access communication system functions as a master of a coexistence control and causes in-home communication systems to coexist with its timing. Therefore, it is relatively easy to achieve QoS in the access service. Also, even when an in-home communication system performs an access control using best-effort CSMA, it is possible to easily control transmission in accordance with an instruction of a coexistence control section possessed by its terminal.